...

NB-EGM-RPI-091409.doc

by user

on
Category: Documents
4

views

Report

Comments

Transcript

NB-EGM-RPI-091409.doc
A Study of Planning Hydrodynamics
Nathaniel D. Barnett,
General Dynamics – Electric Boat
Ernesto Gutierrez- Miravete
Rensselaer at Hartford,
1657 South Rd Kingston RI 02881
[email protected]
275 Windsor St, Hartford CT 06120
[email protected]
Abstract: This paper report on a study of the
hydrodynamics of skimboards and surfboards
using the computational fluid dynamics (CFD)
module in COMSOL. The study analyzes the
flow in a thin water layer underneath a skim
board in a 2-D Cartesian coordinate. Three
different sets of boundary conditions were
employed and one of them produced the best
agreement with previous findings.
Keywords: Fluid Flow, surfing, planning, lift
coefficient, lift force
1. Introduction
A skimboard is a flat board approximately
1-2 m in length and 1m in width that is used to
plane on shallow water along a shoreline for
short distances (10 m max). Boards are normally
used in less than 2 inches (50mm) of water. The
experience is somewhat similar to using a
surfboard, or powerboat, except it takes place in
very shallow water. This study investigates the
flow in the water layer under a skimboard using
the CFD module in COMSOL, and validates the
analysis by comparison with prior work by Tuck
and Dixon [1] and Sugimoto [2]. An analysis of
this type may be helpful to skim board designers
who are interested in prototype testing of design
ideas with fast turnaround time.
Previous studies of skim boards can be found
[1]-[4]. All three studies neglect gravity and
assume one dimensional flow under the board.
In this study, the flow is assumed to take place in
a two dimensional Cartesian system of
coordinates, that the water surface is smooth
surface (no waves) and that the water flow is
negligible. In practice, the individual using a
skimboard uses the board on the flattest part of
the water surface in order to gain the best
advantage.
The skim board user faces tradeoffs during the
operation of the board. The angle of attack
should be as small as possible to reduce drag.
However, the angle must be greater than zero to
remain afloat. The properties of lift disappear
with the introduction of water above the board.
This can be seen in surfing - as soon as the
leading edge digs into the water, the rider takes a
swim. Another tradeoff is the length versus the
potential speed. As the vehicle slows down the
wetted area must increase in order to remain
afloat. Hence, an unpowered skimboard can only
go a few meters from the start point. Sugimoto
also argues that posture and wind resistance pay
a part in the ability to travel on a skimboard. He
concludes that a person standing up straight to
provide the best leverage for balance will forfeit
some advantage with bluff body friction against
the surrounding air. Although the weight of a
board is implicitly taken into account in this
study, no attempt is made to model the wind
resistance.
The main objective of the study was to
compare the previously obtained results using
simplified models, to those obtained using the
CFD module in COMSOL. Another goal was to
demonstrate the development of simple and
easily solvable models to gain meaningful
information about the flow during underneath a
skim board.
A full three dimensional CFD model often
requires months to program, model and solve.
Resolution on a scale that provides useful
information requires an enormous amount of
nodes and corresponding fluid elements. For
example, modeling the rudder and propeller area
of a power boat may require a 3m3
computational box divided up into 30003 nodes,
with the propeller and free surface modeled in
place. For this size a finite element approach
starts to be impractical and finite difference
methods are used. In addition, the free surface
has to be calculated. This requires that special
programming be added into the standard models
available in CFD packages.
A small manufacturing company or a
hobbyist cannot afford the time and resources
required to create such models. They are thus
more likely to make a best “guess” and then
build a prototype to try. For such companies, it
would be desirable to be able to run some quick
calculations over a short period of time to
determine if their design ideas are on the right
track before going to the added expense of
building a prototype. The determination of the
viability of simplified CFD models for the use in
investigating the flow behavior under a planning
type craft, here in the case of a skimboard, can
be very helpful to designers
(2)
The moment about the trailing edge is then
found by multiplying the force with the distance
from the trailing edge, i.e.
(3)
2. Theory and Model Formulation
2.1 Tuck and Dixon and Sugimoto Analyses
[1], [2]:
Figure 1 is a schematic of the system, where a
board is shown standing over a water layer that
moves from left to right to left producing a jet at
the leading edge. This arrangement is entirely
equivalent to the board moving from right to left
over a stagnant water layer.
Both Tuck and Dixon and Sugimoto reviewed a
paper by Edge [3]. Sugimoto compared the
results from Edge paper to those by Tuck and
Dixon paper and concluded that the latter paper
provided a better model for a skimboard.
Sugimoto solved the lift using an alternate
method and obtained agreement with the results
of Tuck and Dixon. Sugimoto
then
went
further and determined the point at which the
skimboard will cease to float and accounted
approximately for wind resistance by assigning a
shape factor of 0.72 for a 70 kg individual and a
bluff body drag coefficient of 1.0.
2.2 Finite Element Model:
Figure 2 shows a sketch of the system
and labels the boundary conditions used in this
study. A two-dimensional Cartesian system of
coordinates is used.
Figure 1: Sketch of the flow under a skim
board [1]
The skim board will be assumed to be a
planing surface of length lw on the water and the
original depth of the water is ho. Moreover, the
board is at a very low angle of attack.
Since the flow upstream of the board tends to
pile up prior to any spray back we can apply
Bernoulli’s equations to the flow. Incorporation
of conservation principles yields the following
expression for the pressure underneath the
skimboard
(1)
Since the jet only covers a minimal portion of
the surface, the lifting force component of the jet
is assumed to be minimal. The lifting force is
then based on the pressure of the wetted area
downstream from the entry point and is given by:
6 7
8
5
1
4
2
3
Figure 2: Sketch of the system being modeled
and description of domain boundaries.
Boundary 1 represents the underside of the skim
board while boundary 4 is the sea floor.
Boundaries 2, 6, and 7 are free surfaces, where
the shape of 7 is based on the geometry given by
Sugimoto. Vertical boundary 5 is the entry
channel and boundaries 3 and 8 are the outflow
channels, respectively, the trailing edge and the
jet. COMSOL Multiphysics was used to create a
model of the above system. COMSOL solves the
governing
equations
for
a
laminar,
incompressible, Newtonian fluid, namely, the
equation of continuity and the momentum
balance (Navier-Stokes) equations.
,
(4)
.
(5)
Regarding boundary conditions, no-slip, nonmoving wall boundary conditions are designated
as u = 0. Free surfaces are assumed to be
symmetry boundaries, and are defined by having
no shear, as in
. For moving walls, the
nodes on the boundary are given the velocity of
that boundary (u= uw). For sliding walls, the
nodes are given the tangential velocity and it is
assumed to have no-slip.
The following specific cases were
considered to determine which scenario
produced best agreement with the results of prior
workers:
 Case 1: With the bottom boundary of the
water layer held fixed, the board is made
to move tangentially along the angle of
attack. The boundary condition in this
case will be as a conveyor belt moving.
 Case 2: Here 2 the board is held fixed
and an entrance velocity is introduced.
The lower boundary is moved with the
same velocity as the incoming fluid, in
order to maintain the relative velocities
of the solid objects. Moreover, the
upper boundary (skim board) is fixed.
 Case 3: The board moves as a rigid body
at constant velocity along the negative x
direction. One problem with this case is
that it fails to take into account how the
board will have lift and float on the
water while the board is in motion above
the terminal velocity, i.e. the point at
which the board does not have enough
lift to rise above the bow wake.
It is noted that through modifications to
the program, such as adding special boundary
conditions for the free surfaces, and special
dimensional changes for board movement, a
much better model could be achieved. However,
since the aim of the project was to verify the use
of simplified models, the above approaches were
considered reasonable.
3. Solution Methodology
The geometry described previously (Figure 2)
was created using COMSOL Multiphysics. The
specific dimensions were selected so as to match,
or closely represent the magnitudes noted in
Sugimoto. The geometry was then meshed using
the generic automatic meshing tool available in
the software. The element shape is of a 2-D
planar tetrahedron with nodes at the apex. The
mesh was made finer with one level of
refinement using the tool available in the
software. The coarse mesh had 366 elements,
and the refined mesh had 1464 elements. A
comparison of calculation time between the two
meshes showed negligible difference in
calculation time and results. Since the purpose of
the project is to show a simple CFD method for
developing planing craft, this general approach
for creating the model was chosen.
Figure 3 below shows the Sugimoto geometry
and the mesh. Note the concentration of nodes
near the leading edge. Since a lot is happening to
the flow in this location the software decided that
added nodes were needed in order to capture the
detail in the flow. It should be noted the shape
of the jet is entered into the geometry prior to
calculation and is not an output of the
calculation. Thus, when using the present
approach, some prior knowledge of the flow is
needed. Calculation of the free surface shape
and the spray jet would require a more extensive
study.
Figure 3: Sugimoto geometry and mesh produced
using COMSOL
The model shown in figure 3 is then solved
using the COMSOL solver with the default
solution parameters and the boundary conditions
noted previously, specifically:

Case 1: The left hand side (boundary #5) is
designated an entryway with a minimal
velocity of 0.3 m/s. The trailing edge (#3)
and the jet (#8) are designated as exits, with
the pressure set to zero. The free surfaces (#
2, 6, and 7) are set to symmetry. The skim
board (boundary #1) is set as sliding wall.
The induced motion (between 3 and 6 m/s)
is that of a conveyor belt.
 Case 2: The water at the entrance channel is
given a velocity equal to that of the lower
boundary (between 3 and 6 m/s). The free
surfaces and exits are given the same
boundary conditions as in case 1. The board
(#1) is held fixed in space with a no-slip
condition.
 Case 3: The board (#1) is then given a
velocity ranging from 3 to 6 m/s in the
negative x direction. A no-slip condition is
then entered for the lower boundary (#4).
The entryway (# 5) is given a velocity of 0.3
m/s. The free surfaces and exit channel are
given the exact same boundary conditions as
cases 1 and 2.
Steady state conditions were assumed
for all solutions. This was justified since the
velocity field changes little from one small
fraction of a second to the next.
Computed velocities, streamlines, pressure
distributions and the like were all easily
generated by the program and they were useful
in determining the appropriateness of the
selected boundary conditions. Other postprocessing tools available in the software were
then used to investigate the computed field
variables along selected cut lines. Specifically,
the pressure distribution under the board was
plotted against the length of the board. The
computed pressure distribution was then
integrated using a trapezoidal method in order to
obtain the lift forces. The computed forces were
finally compared against the results obtained by
prior workers.
4. Results
The COMSOL finite element model was run for
the three cases mentioned above and the results
obtained in each case were carefully examined
for comparison with previous solutions. Figure 4
shows an example of the computed flow for an
induced velocity of the board at 3m/s with the
boundary set as a sliding wall (case 1). The axes
of the velocity contours are distances in meters.
The model clearly shows the action of the flow
around the jet. It should also be noted that when
the board is moved in this manner in the model
the fluid tends to move with the board, dragged
along by the no-slip condition at the boundaries.
Figure 4: Velocity streamlines: Case 1
The corresponding pressure profile on
the underside of the skimboard is shown in
figure 5. In addition to the profile graph, the
software can export out data points. These points
can easily be entered into a spreadsheet or
similar program to integrate and find the total lift
force on the wetted area. The same line of
pressure was used in all of the CFD cases. The
pressure found is assumed to be the pressure on
the underside of the board. The data was
obtained from points in space very close (less
than 0.005 m) to the nodes on the board.
Therefore, very little error is induced as the
program will output a proportional average of the
nearest nodes. Thus the great majority of the data
comes from the nodes touching the boundary, or
adjacent to it.
The computed pressure profile is shown
in figure 5. When this function is integrated
numerically, the resulting lift force is 584N,
which is enough force to provide lift for a 60 kg
individual. The sliding wall boundary condition
assumes that the wall is moving tangentially, in
the direction of the angle of attack. While this
may be the case occasionally while riding a
skimboard, it does not produce similar results to
the theoretical analysis. The small blip near the
leading edge of all three cases appears to be an
artifice of the model.
Figure 7: Pressure profile (Pa): Case 2
Figure 5: Pressure profile (Pa): Case 1
4.2 Case 2:
The streamlines calculated in this case
are shown in figure 6 while the pressure profile
can be seen in figure 7. Note that the pressure
never drops below zero (atmospheric nominal)
using the second method. Also, the center of
pressure can be seen to be more towards the
center of the board. The lift force using this
method almost doubles to 1100 N. This is closer
to the values from Tuck and Dixon, but still
underestimated by a factor of four. The
streamlines near the leading edge match closely
those obtained by Tuck and Dixon.
Figure 6: Velocity Streamlines: Case 2
4.3 Case 3:
Figures 8 and 9 show respectively the
resulting streamlines and pressure distribution
obtained in this case. The spray jet can be seen
to be developed with a divided flow. The
pressure curve was found to be more uniform.
Integration of the pressure yielded the total lift
force to be 2821N, much closer to the expected
values from Dixon. Based on the streamline plot,
it appears as though the flow is almost sheared
off by the dimension of the jet. The streamlines
do not match those from the Tuck and Dixon
paper. The same model was also run for 4 and 5
m/s. For 4 m/s the lift is 3715N and for 5 m/s it
is
4622
N.
Figure 8: Velocity streamlines: Case 3
2500
2000
pressure
1500
1000
500
0
0.00
-500
0.50
1.00
1.50
2.00
distance along board
Figure 9: Pressure profile (Pa): Case 3
5. Conclusions
Useful information about the flow underneath
skim boards can be obtained from models readily
built using the CFD module in COMSOL
Multiphysics. Of the three cases considered in
this study, only in the third it was found that the
lift generated by the computational methods was
in good agreement with the lift values noted in
previous theoretical studies. Case 1 was found to
produce the lowest lift, note quite enough to lift a
70 kg individual. Since a 70 kg person has been
witnessed and observed on a board this solution
was discounted.
Some knowledge of the particular flow and
forces was vital in being able to determine the
viability of the data obtained. Without the benefit
of the previous studies it may have been difficult
to determine which of the boundary conditions
produced the best results. Certainly from an
aesthetic standpoint case 2 produces the prettiest
picture. However, the forces in this case were not
the closest match to the theoretical. Observations
in the field show that the closest representation
to the flow is case 3.
Although some error is introduced in
both the computational and empirical methods,
for a designer working on a project, the
visualization of the flow using commercially
available software can be an invaluable tool to
assist with rapid prototyping of design concepts.
6. References
1. Tuck, E.O. Dixon, A. Surf skimmer
hydrodynamics. Journal of Fluid Mechanics.
205, 581-592, 1 February 1989.
2. Sugimoto, Takeshi. Mechanics of a surf
skimmer revisited. American Journal of
Physics. 71,144-149, February 2003.
3. Edge, R D. The surf skimmer, American
Journal of Physics. 36, 630-631, July 1968.
4. Green, A.E. The Gliding of a plate on a
stream of finite depth. Proceedings of the
Cambridge Philosophical Society. 31, 589603, 1936.
Fly UP